A user owning several analogue telephones all assigned to the same telephone number and connected in parallel e.g. to the Plain Old Telephone System (POTS) can easily hand over a call from one interconnected (here: engaged in a call or a session) telephone to another telephone currently not interconnected simply by off hooking the other telephone. That means in case of analogue telephones, features can be provided even for telephones that are currently not interconnected by means of very simple user actions like e.g. off-hooking the telephone.
With data packet networks like IP networks (IP=Internet Protocol) becoming more and more common, the use of data-packet-based telephones like e.g. the so-called SIP telephones (SIP=Session Initiation Protocol) that apply SIP as signaling protocol is strongly increasing. However, two SIP telephones can not be configured and applied in a way that they behave as two analogue telephones connected in parallel. Applying the SIP protocol specifications, it is e.g. not possible that a SIP telephone is interconnected in parallel to an already existing interconnection simply by off hooking the SIP telephone, i.e. for a SIP telephone, no features can be provided by means of very simple user actions as it is the case for analogue telephones. Therefore, additional actions by the user, as e.g. pressing a sequence of buttons, are required for the handover of a call from one SIP telephone to another SIP telephone, with both telephones being assigned to the same telephone number. SIP telephones expect e.g. that the user always presses a kind of enter button at the telephone, if the dialed number shall be transmitted or an interconnection shall be established. For non-technical persons or for persons who are used to hook off another telephone in order to continue the call, it is not acceptable to first study a technical description e.g. to learn the procedure to handover the interconnection to another telephone. This procedure might even be interpreted by the user as a malfunction.